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SDD Epitaxy Type
Brand new epitaxy type
In 2005 KETEK engineers presented a further quantum step in detector technology by replacing ion implantation doping with selective silicon epitaxy (see: publications). The principle process flow for fabrication of the next generation of drift detectors is briefly explained in the following:
In a first step the high purity silicon wafers are passivated by an oxide layer with extremely low interface state density and low oxide charge, necessary to minimize surface leakage current. This oxide growth is one of the basic key technologies for detector device fabrication. Subsequently oxide windows are opened by lithography and wet chemical etching on the anode side to define the highly doped anode region.
During the next step the anode is deposited by selective epitaxial growth (SEG) in a LPCVD epitaxy reactor into the oxide window.
The remaining p+n- junctions for drift field and entrance window are generated in two back-to-back steps of 1. oxide structuring, 2. boron doped SEG-processing and 3. deposition of passivation oxide.Therefore at last all deposited silicon areas are covered by oxide layers.
Finally the contact holes are opened and the metallization is deposited and patterned on both wafer sides to enable contacting by ultrasonic wire bonding for the packaging of the chip.
This unique KETEK technology also enables the realisation of large area silicon drift detectors with low capacitance anodes and small leakage currents as the implantation technology does. However, eptitaxy is superior to implantation doping in respect to crystal defects, which are the source of noise both in the detector and in the integrated FET. Integration of electronic components by the use of epitaxy results in better noise behaviour of the FET as well as it avoids any influence of detector biases on the electronics. Last not least, by epitaxy it is possible to configure extremely shallow and radiation hard entrance windows for low energetic x-rays.
